I had to look back at the original article from Mozilla Links. According to the article, the new behaviour would be “will be featured in future releases.” Also, “the new behavior will be added to trunk builds soon, but its inclusion in Firefox 3.1 is yet to be decided.” I’ve been using the trunk builds throughout the Firefox 3.1/3.5 cycle and don’t recall seeing it included. I have a feeling that Private Browsing and getting TraceMonkey functional and stable took higher priority over this tab behaviour. Doubtful this is going to make the Beta 4 released based on reading about this in BugZilla (Bug 465673).